The present invention relates to a system for controlling air-fuel ratio for an automotive engine mounted on a motor vehicle so as to prevent fluctuation in engine speed while the engine is idling.
Many air-fuel ratio control systems with a three-way catalytic converter have been proposed for cleaning exhaust gases. The control system is provided with an O.sub.2 -sensor and a feedback control system which performs a feedback operation using an output signal of the O.sub.2 -sensor to control the air-fuel ratio to the stoichiometric air-fuel ratio which is most effective for the three-way catalytic converter for purifying the exhaust gases. In such a system, the air-fuel ratio cannot be fixed to a constant value, but varies in the vicinity of the stoichiometric air-fuel ratio. Therefore, under the idling condition of the engine, the engine speed fluctuates as the air-fuel ratio varies, causing problems such as unstable fuel combustion.
FIG. 1 shows a PI signal from a proportion and integration (PI) circuit in the feedback control system, the air-fuel ratio, and the engine speed which varies in accordance with the variation of the air-fuel ratio.